In hemodialysis treatments using an artificial kidney it is necessary, in the interest of patient safety, to monitor the positive pressure of the blood being returned to a patient's vein and to insure that the returning purified blood is free of particulate matter and gas bubbles. Heretofore, it has been conventional to perform the blood pressure measuring step by incorporating a venous drip chamber in the blood tube that is connected to the patient's vein.
Typically the venous drip chamber is secured to a stand or support adjacent the patient such that it remains upright during the treatment to insure the rise of gas bubbles to the top portion of the chamber. The bubble chamber serves the dual function of bubble removal and of providing a site for measuring the pressure of the blood in the return tube path to the patient's vein. The drip chamber is a closed receptacle and as pressure changes or separated bubbles add to the air space at the top of the chamber it is necessary, periodically, to inject a needle into the air space and suck out some of the gas to maintain a preset level in the chamber to avoid the possibility of air bubbles reaching the patient and causing a fatal embolism.
There are a number of undesirable aspects to the use of such venous drip chambers. First, repetitive needle penetrations increase the potential of creating a non-sterile circuit. Second, relatively constant observation of the blood level by the clinic attendant is required and personal withdrawal of excess gas requires time and effort during the normal four to six hour hemodialysis treatment. Third, there is a continuously existing blood-air interface within the drip chamber and the exposure of a patient's blood to air during the extended four or more hours during the hemodialysis treatment tends to degrade, contaminate, denature, or even clot the blood in the chamber. For this reason, a need for an airless artificial kidney system has been recognized since at least the early 1970's as hollow fiber artificial kidney use increased. FIG. 10 of U.S. Pat. No. 4,231,871 suggests the use of a microporous vent and blood pressure measuring means located in the venous line without showing a specific construction of either unit.
It was found that microporous vents having the form of a disc mounted at the top of a tubular shaped filter device, as shown in FIG. 10 of U.S. Pat. No. 4,231,871, had two operational problems. First, when using a hydrophobic material such as polytetrafluoethylene, having micro-sized openings in the range of about 1 to about 30 microns in the vent disc, clogging of the small openings with blood platelets occurred as the time of use extended and on occasion there was some foaming and some clotting of the blood adjacent the lower surface of the hydrophobic vent. Second, it was found that operating conditions which placed a negative pressure on the lower surface of the vent disc caused air to be drawn through the vent and into the blood chamber. The improved microporous vent containing subassembly of this invention overcomes both of these problems and provides an improved airless operating system, as will be explained in detail hereinafter.
Microporous vents per se and certain constructions using microporous vents to remove air, or entrained gases, from blood or other liquids prior to, or during, intravenous injection into a patient were known prior to this invention. Hydrophobic microporous membranes are shown in U.S. Pat. Nos. 3,778,971 and 3,993,062 and a combination of a hydrophilic and a hydrophobic separator is shown in U.S. Pat. Nos. 3,854,907, 4,004,587 and 3,523,408. These constructions employ tubular separator configurations, pouch-shaped devices as well as combinations of cylindrical separators with disc shaped separating membranes. The problem of ambient air entering into a gas separating filter is recognized in U.S. Pat. No. 4,190,426 and a variety of mechanical check valve constructions have been developed to overcome that problem and are discussed in a number of U.S. patents described in columns 1 and 2 of U.S. Pat. No. 4,190,426, which discussion is hereby incorporated herein.
The microporous vent construction of this invention employs a special housing configuration that includes only a hydrophobic separator and a novel, non-mechanical means to prevent the entry of ambient air into the filtering chamber.
In the past, measurement of blood pressure in the venous blood tube was accomplished by connecting a pressure transducer to the air space above the blood in the venous drip chamber since the pressure on the air in that space is the same as the blood pressure in the same chamber. As above stated, elimination of the blood-air interface is desirable and this invention employs pressure measuring means which does not require air, or gas of any composition, to interface with blood in the blood return path to the patient. Rather, blood pressure measuring means and the microporous vent are combined in a single tubular housing together with a blood filter that during hemodialysis operates completely filled with blood and free of air or other gas. The blood pressure measuring means employs a compressible diaphragm in a spherical or cylindrical receptacle mounted into the wall of the housing such that the diaphragm contacts the blood flowing through the housing as it returns to the patient. The blood pressure measuring receptacle contains air isolated from the blood in the housing by the compressible diaphragm. Movement of the diaphragm responsive to the pressure on the blood in contact with it in the housing expels air from the receptacle which is connected to a remotely located pressure indicator precalibrated to reflect blood pressure. Pressure detecting and measuring devices which include a deformable element having the shape of bellows, truncated cones, hemispheres or a diabolo are shown in U.S. Pat. No. 3,554,035. A frusto-conical, thin membrane disposed in a housing which transmits blood pressure variations through a pressure transmitting medium to a pressure transducer is shown in U.S. Pat. No. 4,077,882. Pressure transducers which employ flexible diaphragms have been used as gauges for gasoline or oil in U.S. Pat. No. 2,385,382, for sterile fluid measurements as shown in U.S. Pat. No. 3,818,765, and for blood as shown in U.S. Pat. No. 3,713,341. Pressure transmitting means responsive to pressure activated diaphragm elements include various fluids such as air, mercury, gasoline, etc., as shown in U.S. Pat. Nos. 2,369,707 and 3,349,623, or mechanical means as shown in U.S. Pat. No. 2,272,950. The above identified prior art represents the most pertinent art known to applicant relating to the separate microporous vent and diaphragm actuated pressure measuring elements which are satisfactory for use, in modified form, in the subassembly of this invention.
In addition to the automatic deaerating, blood pressure measuring, filtering subassembly of this invention, the overall airless artificial kidney assembly includes an artificial kidney and a blood tube for supplying blood from a patient's artery to the kidney and a blood tube for returning blood from the blood pressure measuring, air venting, filtering subassembly to the patient's vein. By virtue of combining the functions of automatic bubble separation and blood pressure measuring into a combination means that eliminates the need for a blood-air interface to enable pressure determination, it becomes possible to eliminate the conventional venous drip chamber as a part of the blood tubing set. Conventionally, blood tube sets have also included injection sites for heparin administration and blood sampling sites, which permit needle insertion through the blood tube wall. Constructions of such sites that assure safety to the nurse or technician using same are shown in shown U.S. Patents as Nos. 4,184,489, and 3,447,570. The direct, rigid attachment of the subassembly of this invention to the artificial kidney permits elimination of such separate blood tube site constructions by the incorporation of one or more of such sites into selected, accessible wall surface locations of the subassembly housing. The subassembly of this invention does include at least one such site. The elimination of the venous drip chamber and access sites from the blood tube set makes it feasible to flush and clean the blood tubes as well as the artificial kidney after a hemodialysis treatment to a degree of cleanliness that enables safe reuse of the blood tubes and the artificial kidney, whereas prior practice required discarding the entire blood tubing set. It remains desirable to discard the subassembly, or portions thereof, after a single use and to replace it, or the portions, with a substitute.
To the best knowledge of applicant, the assembly of this invention is the first artificial kidney assembly which has enabled the cleansing of a patient's blood in an extracorporeal circuit that is airless and free of a blood-air interface at any location in the extracorporeal circuit. It is also the first such assembly that has provided the option of safe reuse of the blood tubing as well as the artificial kidney.